This application is based on Japanese Patent Application No. 2000-15939 filed Jan. 25, 2000, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a method and an apparatus for holding micro-balls in a plurality of pits, respectively, opened on the surface of a ball carrying pallet. Particularly, the present invention is suitably applied to solder balls for forming electrode bumps on a circuit board or the like.
2. Description of the Related Art
When solder balls are placed on a bump-forming portion in a semiconductor chip or a circuit board coated in advance with flux, and molten to form electrode bumps, it is necessary to accurately position the solder balls of a micro size must be to the bump-forming portion in the semiconductor chip or the circuit board. For this purpose, a template or a ball carrying pallet is used, having a plurality of pits arranged thereon in the same pattern as that of the electrode bumps to be formed in the objective. That is, there has been known a method in which the solder balls placed in the pits of the ball carrying pallet are attractively held at once by a suction head with vacuum, and after the suction head has been located directly above the objective such as a semiconductor chip or a circuit board so that the lower end of the respective solder ball attractively held in the suction head is brought into contact with the flux on the bump-forming portion, the solder balls are released from the suction to transfer onto the flux in the bump-forming portion. Such a method is disclosed, for example, in Japanese Patent Application Laid-open No. 5-129374 (1993).
In this case, it is necessary for the solder balls to be held neither too much nor too less in the pits of the ball carrying pallet. In general, it is not so easy to hold the solder balls on the ball carrying pallet nether too much nor too less since the solder ball is extremely small in size.
To solve this problem, Japanese Patent Application Laid-open No. 11-8272 (1999) proposes a technology wherein a ball carrying pallet formed of silicon wafer is immersed in ethanol, and solder balls are made to drop onto ball carrying pallet through the ethanol, then rolled thereon to get into the individual pits. This method is advantageous in that the ball-carrying pallet can be quickly and easily dried due to the high volatility of ethanol, whereby the delay of subsequent process is avoidable.
On the other hand, a dry method for lining up the solder balls in air is problematic in that the solder balls are stuck to each other or to the surface of the ball carrying pallet due to static electricity or moisture, resulting in the difficulty in the stable lining-up operation.
Contrarily, an aforementioned wet method disclosed in Japanese Patent Application Laid-open No. 11-8272 (1999) is advantageous because it is possible to stably line up the solder balls in the pits of the ball carrying pallet while avoiding the adverse effect due to static electricity or moisture generated in air. It requires, however, the frequent replenishment of ethanol because of the high volatility of the latter. Further, a large amount of ethanol is necessary for continuing a stable operation. Also, since the ball-carrying pallet is in general small in thickness, the handling thereof in the immersed state in ethanol is considerably difficult.
An object of the present invention is to provide a method for lining up micro-balls, capable of suppressing the consumption of highly volatile liquid carrier used in a wet method as well as of easily holding the micro-balls in pits of a ball carrying pallet, and an apparatus for carrying the method.
A first aspect of the present invention is a method for lining up micro-balls in a plurality of pits, respectively, formed on surfaces of ball carrying pallets, comprising the steps of:
defining a first sealed chamber by attaching the ball carrying pallets to a lining-up container so that the surfaces of the ball carrying pallets abut on the chamber,
supplying the micro-balls dispersedly mixed with liquid carrier stored in a second sealed chamber to the first sealed chamber via a flexible duct so that the micro-balls are held in the pits of the ball carrying pallets, respectively,
returning the surplus solder balls together with the liquid carrier from the first sealed chamber to the second sealed chamber via the duct, and
removing the ball carrying pallets from the lining-up container.
The method for lining up micro-balls according to the first aspect of the present invention may further comprise the step for vibrating the ball carrying pallets while supplying the liquid carrier in which are dispersed the micro-balls to the first sealed chamber.
The transfer of the micro-balls and the liquid carrier between the first and second chambers may be carried out by shifting the relative positions of both the sealed chambers in the vertical direction.
The liquid carrier may be electrically conductive. In this case, the liquid carrier may be alcohol. If the liquid carrier is electrically conductive, such as alcohol, the micro-balls can be more assuredly held in the individual pits, and further the ball-carrying pallet can be quickly dried while holding the micro-balls in the pits.
The micro-balls may be solder balls.
A second aspect of the present invention is an apparatus for lining up micro-balls, comprising
ball carrying pallets, each having a plurality of pits for holding the micro-balls, respectively, on its surface,
a pallet holder for holding the ball carrying pallets,
a lining-up container defining a sealed chamber in association with the pallet holder hermetically fitted thereto,
an storing tank for storing liquid carrier in which micro-balls are dispersed, and
applying/collecting means for communicating the storing tank with the lining-up container via a passage to supply the micro-balls together with the liquid carrier from the storing tank to the sealed chamber and return the surplus micro-balls together with the liquid carrier from the sealed chamber to the storing tank.
In the present invention, the pallet holder carrying the ball carrying pallets is hermetically fitted with the lining-up container to define the first sealed chamber. In this state, the micro-balls in the storing tank defining the second sealed chamber are fed by the pumping means to the first sealed chamber together with the liquid carrier so that the micro-balls get into the individual pits formed on the surface of the ball carrying pallets. Then, surplus micro-balls are returned from the first sealed chamber to the storing tank together with the liquid carrier by the pumping means, after which the pallet holder together with the ball carrying pallets carrying the solder balls in the respective pits is removed from the lining-up container defining the first sealed chamber now having no liquid carrier therein.
According to the present invention, it is possible to suppress the consumption of highly volatile liquid carrier and stably carry out the operation for lining up the solder balls in the pits of the ball carrying pallets because an amount of liquid carrier is maintained substantially constant. Also, since the ball carrying pallets are mounted to the pallet holder which in turn is detachably mounted to the lining-up container, the handling of the ball carrying pallets is extremely enhanced, whereby the lining-up operation can be automated to facilitate the working efficiency.
An apparatus for lining up micro-balls according to the second aspect of the present invention may further comprise vibration means for vibrating the pallet holder while the latter is fitted to the lining-up container. In this case, the micro-balls can be quickly and assuredly held in the respective pits.
The applying/collecting means may transport the liquid carrier by changing a position of the storing tank in the vertical direction relative to the lining up container fitted with the pallet holder. In this case, there is no need for using a pump having a high probability of blockage due to micro-balls.
The applying/collecting means may operate to incline, relative to the horizontal plane, the surfaces of the ball carrying pallets held by the pallet holder fitted with the lining-up container. In this case, an amount of liquid carrier remaining on the surface of the pallet can be limited as small as possible to minimize the consumption thereof.
The micro-balls may be solder balls.
The above and other objects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.